Introduction
All RAS rely on the common biological treatment process – nitrification – to avoid accumulation of ammonium. The process takes place in biofilter and has numerous assets and drawbacks. An alternative to autotrophic biofilters is heterotrophic N assimilation (HET-N) (De Schryver and Verstraete, 2009; Ebeling et al., 2006). In this process, heterotrophic bacteria consume ammonia directly for growth and thereby remove dissolved N. HET-N assimilation require a higher carbon to nitrogen ration (C:N ratio) which can be achieved by adding a bioavailable carbon source – i.e. acetate (OA) to the system. This can be done by adding easily bio-degradable carbon in the water to promote bacterial growth in the water. However, the addition of carbon and corresponding growth of heterotrophic bacteria leads to a large increase in oxygen consumption, increase in turbidity and high suspended solids loads (Crab et al., 2012). An alternative carbon source is slower releasing products like polyhydroxybutyrate (PHB) (Luo et al., 2020). These PHB pellets placed in a reactor may reduce the turbidity and suspended solids compared to direct carbon addition. The objective of this study was to compare traditional RAS with nitrifying biofilter with different modes of carbon addition to obtain heterotrophic N assimilation. The main focus included assessment of changes in water quality and implication on fish performance during a 4-week experimental period.
Materials and methods
Four different treatment groups were tested in triplicate in 1.7 m3 pilot scale RAS:
1) RAS with biofilter (Control); 2) RAS with biofilter + acetate addition (BF+OA); 3) RAS without biofilter + addition of acetate (OA only) and 4) RAS without biofilter + PHB biopellets (PHB only).
Water samples were collected weekly during the duration of the trial and tested for different physical, chemical and biological parameters. Fish survival was compared and performance was assesses by weight gain during the trial.
Results and discussion
Control systems led to accumulation of NO3 as predicted during the duration of the trial (Fig 1a). In contrast, all other RAS with carbon addition were found to have much lower levels of nitrate at the end of the trial, with 70% less NO3 in the PBH only RAS. Despite not having any biofilters installed, both OA only and PHB only RAS maintained acceptable water quality parameters during their respective start up phases. The OA only RAS showed a moderate accumulation of NH4+ during the first 2 weeks (up to 0.747 mg l-1 TAN), and a constantly higher level of NO2 throughout the trial (0.834 mg Nl-1 vs approx.. 0.100 mg N l-1 in the control group). The PHB only RAS did not cause accumulation of TAN and only a moderate increase in NO2- in the first week. This indicates a very fast start up time for Het-N systems, which could have important monetary implication for fish farms as it reduces start up time and allows for an increase feeding from earlier in production. The addition of acetate to the water (BF+OA and OA only) caused expected formation of bioflocs in the systems, and a significant increase in bacterial activity and turbidity (fig. 1b and 1c). While the fish in all tanks showed apparent appetite and readiness to eat, tanks where acetate was added (OA only) led to substantial feed spill. The feed spill may have been caused by the deteriorating water quality conditions affecting fish appetite, or could simply be due to visual impairment of the fish, reducing their ability to ingest the feed. In contrast, system fitted with biopellets (PHB only) remained stable and showed no changes in both physical (turbidity) and biological (bacterial activity) parameters during the trial. The absence of bacterial accumulation and no increase in turbidity in the water in PHB only suggests that the processes primarily occurred within the reactor, having no negative impacts on water quality.
These results translated to differences in the growth of the fish during the trial, with no differences found between the control groups and the biopellet group (fig. 1d), but a reduction in weight gained in both acetate groups (7,4% less growth in the BF+OA and 21 % less growth in the OA group). Survival was over 99%.
The results of this trial indicate that it’s possible to convert a conventional RAS to Het-N assimilation through the use of different carbon sources, without compromising fish survival and reducing dissolved N and P in the water. However, the use of fast degrading carbon such as acetate resulted in deteriorating water quality and affected fish performance by significant reduced growth. In contrast, slow degrading carbon such as PHB biopellets in a reactor, resulted in reduced levels of N and P, with none to negligible effects on water quality and equal fish performance, while having a much faster start up time compared to traditional autotrophic biofilters.
References
Crab, R., Defoirdt, T., Bossier, P., Verstraete, W., 2012. Biofloc technology in aquaculture: Beneficial effects and future challenges. Aquaculture 356–357, 351–356. https://doi.org/10.1016/j.aquaculture.2012.04.046
De Schryver, P., Verstraete, W., 2009. Nitrogen removal from aquaculture pond water by heterotrophic nitrogen assimilation in lab-scale sequencing batch reactors. Bioresour. Technol. 100, 1162–1167. https://doi.org/10.1016/j.biortech.2008.08.043
Ebeling, J.M., Timmons, M.B., Bisogni, J.J., 2006. Engineering analysis of the stoichiometry of photoautotrophic, autotrophic, and heterotrophic removal of ammonia-nitrogen in aquaculture systems. Aquaculture 257, 346–358. https://doi.org/10.1016/j.aquaculture.2006.03.019
Luo, G., Hou, Z., Tian, L., Tan, H., 2020. Comparison of nitrate-removal efficiency and bacterial properties using PCL and PHBV polymers as a carbon source to treat aquaculture water. Aquac. Fish. 5, 92–98. https://doi.org/10.1016/j.aaf.2019.04.002